Metal plating of a substrate is used in many diverse and ever expanding fields, including the manufacture of printed circuit boards, mirrors, conductors, textiles, and decorative automobile parts. As a consequence there is considerable interests in the development of a variety of metal plating processes designed to meet the needs of the various users. With respect to the manufacture of conductors, it would be particularly desirable to replace twisted tinsel wire which is relatively weak and noisy to manufacture with a metal plated substitute that would be stronger and less noisy to manufacture. However, a major obstacle to metal plating of substrates is the catalyzation of the surface so that it can then support subsequent electroless plating. In response to the problem, several processes to prepare the surface of a substrate for metal plating have been developed. One such approach has been to immerse the substrate in an acidic sensitizing solution which becomes absorbed onto the surface and will reduce the salt of a precious metal catalyst to the corresponding catalytic metal. Thereafter, the sensitized substrate is immersed in a bath containing the salt of a precious metal catalyst. The catalytic metal becomes deposited on the surface of the substrate and provides a plurality of discrete nucleation centers fo subsequent electroless plating of a conductive metal such as copper, chromium, or nickel. A typical sensitizing solution includes stannous chloride, and a reducing agent. The precious metal salt is typically palladium chloride or silver nitrate (U.S. Pat. Nos. 3,518,067; 2,010,805).
One of the problems of the above mentioned method is insufficient adhesion of the electroless plated metal onto the substrate. In order to increase adhesion, several pretreatment steps have been recommended. For example, the surface can be etched with a strong oxidizing solution such as chromic acid/sulfuric acid or with N-N -dimethyl formamide, pyridine and substituted pyridine compounds of alkylene glycol that are soluble at pH 8.7 to 10.7 (U.S. Pat. No. 3,518,067). In addition, it has been recommended that the surface be swelled and softened with organic solvents such as aliphatic alcohols and ketones including methanol, ethanol, acetone, and methyl ethyl ketone (U.S. Pat. Nos. 3,533,828; 3,574,070).
Another approach to activating dielectric surfaces is to treat the article with a silver salt dissolved in an evaporable organic solvent. The salt is then reduced to electro-conductive silver by immersion in an alkaline reducing bath of NaOH, formalin and water. A solution of silver perchlorate in benzene or toluene has been used as the source of silver (U.S. Pat. No. 2,511,472).